System for flexible multiple broadcast service delivery over a WDM passive optical network based on RF block-conversion, of RF service bands within wavelength bands

ABSTRACT

A system and method for simultaneous delivery of a plurality of independent blocks of 500 MHz digital broadcast television services, stacking a plurality of RF blocks on a plurality of spectrally sliced WDM optical bands. The method for delivering a plurality of video blocks to a user terminal serviced by a remote node comprises the steps of receiving, by a first WDM, a broadband signal from a broadband signal source, separating, by the first WDM, the broadband signal into a plurality of optical hands, modulating each of the plurality of optical bands with a composite signal representing data in a plurality of independent RF blocks to form a plurality of modulated signals, forwarding the plurality of modulated signals to a second WDM to form a combined broadcast signal, transmitting the combined broadcast signal over feeder fiber to a remote node, selecting a RF block for distribution over a distribution fiber to a conventional satellite set-top box at a user&#39;s site and forwarding the selected RF block to the user&#39;s site. A novel method and system for reducing spontaneous beat noise is also described.

This application is a continuation and claims the benefit of U.S.application Ser. No. 11/448,297, filed Jun. 7, 2006 now U.S. Pat. No.7,466,919 which is a division of U.S. Ser. No. 09/916,652, filed Jul.30, 2001, now U.S. Pat. No. 7,085,495, which claims the benefit of U.S.Provisional Appl. Ser. No. 60/223,873, filed Aug. 8, 2000.

FIELD OF THE INVENTION

The invention relates generally to Wavelength Division Multiplexing(WDM) passive optical networks (PONs) and specifically to the use ofsuch networks for the simultaneous delivery of multiple RF bands, eachnominally equivalent to a conventional CATV service, thereby increasingcapacity and flexibility of such systems.

BACKGROUND OF THE INVENTION

Telecommunications services generally fall into two major categories:broadcast services, in which all user terminals receive the sameinformation, and switched services, in which each user terminal receivesinformation specific to him/her. Network infrastructures can besimilarly classified. An example of a broadband infrastructure is aconventional CATV network, while an example of a switched infrastructureis the public switched telephone network (PSTN). In general, it is morecost effective to deliver broadcast services on broadcast networks andswitched services on switched networks.

While the thrust of the present invention is broadcast telecommunicationservices, and thus, typically one-way service, broadcasttelecommunication services can also include two-way services forinteractive services e.g., shopping and games and may include audio aswell as video. Two-way services may include information, interactiveshopping and games and other services. The upstream portion of thetelecommunications would be provided in a known manner and notnecessarily in the manner described for the present invention. Upstreamservices could, however, be combined with the downstream broadcastservices of the present invention described herein.

Recent work has shown that the optical properties of certain passivedevices can be exploited to permit a given infrastructure to emulateproperties of both switched and broadcast infrastructures and thusefficiently provide both switched and broadcast service. In particular,the cyclical properties of Waveguide Grating Routers (WGRs) used inconjunction with Wavelength Division Multiplexing (WDM) provide flexiblepartitioning of both types of networks using the same physicalinfrastructure. WGRs are also sometimes called Arrayed Waveguide Grating(AWG), Phased Array (Phasar) or Dragone Routers. The WGR acts as thedistribution element at the remote node. Any reference, therefore, to aWGR should be deemed to encompass any such device or other known devicefor performing similar functionality.

Much work has been done recently to demonstrate the possibilities ofsuch a system to deliver large quantities of digital TV carriers. Aparticularly robust QPSK (quadrature-phase-shift-keying) transmissionformat permits the use of low quality and potentially inexpensiveoptical sources with wide optical bandwidths. In particular, it has beenshown that the wavelength domain can be used to deliver “blocks” oftelevision programming channels (one RF block per wavelength band) in asimilar manner to known analog block converters. These demonstrationshave been shown to deliver multiple 500 MHz blocks of QPSK modulatedcarriers of, for example, television channels from a Satellite serviceprovider, using the location of the optical band as a multiplexingindex.

SUMMARY OF THE INVENTION

The WGR permits simultaneous and cost-effective transmission of bothbroadcast and switched services with tremendous flexibility.Wavelength-specific lasers are used for high-speed point-to-pointswitched connections, while broadcasting uses broadband sources (LightEmitting Diodes (LEDs) or Amplified Spontaneous Emissions (ASE)sources), which illuminate all output ports of the WGR at once. Opticalbandpass filters spanning a free spectral range (FSR) of the wavelengthcyclic WGRs define cascadable service bands.

Using the cyclical or periodic properties of the WGR together withoptical sources having wide optical spectra, favor broadcast delivery,while “line sources” with narrow spectra favor switched servicedelivery. The use of a wide optical spectrum floods the output opticalchannels so that each output port carries a replica, or spectral slice,of the signal on the input port. The linear properties of this passivedevice make it possible to overlay both types of services simultaneouslyon the same infrastructure. The ability to segregate such services hasbeen termed “WDM-on-WDM” in recognition that a coarser scale of WDM (onthe order of the period, or “free spectral range” of the WGR) can beused to segregate a multiplicity of both broadcast and point-to-pointservices on an intrinsically “dense WDM” infrastructure traditionallysuitable for point-to-point switched services.

The present invention further increases the capacity (and flexibility)of the QPSK systems capable of delivering blocks of programming byfrequency-division multiplexing multiple RF blocks onto each wavelengthband. The user terminal may access this large video content by using aconventional satellite set-top box with a front end that has an opticalfilter for selecting the appropriate optical bands and an RF converterto select the appropriate RF carrier blocks.

The present invention, thus, multiplies the capacity of a known systemby increasing the number of RF blocks for each wavelength band. The vastnumber of RF subcarriers, however, may drive the spectrally slicedchannels at the receiver into a spontaneous-spontaneous (sp-sp) beatnoise limited regime. Our novel method using multiple WGR input ports toexpand the effective optical bandwidth of the received signal enablesthe present invention to higher capacity than would be possible with themore conventional method of using a single WGR input port. Data arepresented showing the simultaneous operation of the entire servicematrix of wavelength and RF bands containing 1280 video channels.

It is therefore an object of the present invention to increase thecapacity and flexibility in the delivery of broadcast and switchedservices to a user terminal serviced by a remote node.

It is yet another object of the present invention to increase thedelivery of blocks of programming by frequency division multiplexingmultiple RF blocks onto each wavelength band.

It is a further object of the present invention to contain costs byusing a front end coupled to a conventional satellite set-up box. Thefront end has an optical filter and an RF converter.

It is yet another object of the present invention to reduce thespontaneous-spontaneous beat noise by using multiple WGR input ports.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best described with reference to the detaileddescription and the following figures, where:

FIG. 1 shows a sample frequency plan for WDM and RF multiplexedservices.

FIG. 2 a depicts the system set up for generating thewavelength-frequency plan of FIG. 1.

FIG. 2 b shows the four independent RF blocks that are summed.

FIG. 2 c depicts an alternative embodiment of the present invention.

FIG. 3 a depicts the system concept of service multiplexing using WDMand RF carrier stacking.

FIG. 3 b shows an alternative embodiment of the user site portion of thepresent invention.

FIG. 4 shows how the cyclic frequency (or periodicity) property of theWGR is used to deliver multiple optically segregated services to eachuser.

FIG. 5 shows the receiver sensitivity for RF subcarriers in optical band2.

FIG. 6 illustrates receiver sensitivity for subcarriers 1 and 16 invarious optical bands and RF blocks.

FIG. 7 shows the power penalty due to sp-sp beat noise as a function ofthe effective optical bandwidth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a sample frequency plan for WDM and RF multiplexedservices. Four wavelengths and four RF blocks per wavelength are used inFIG. 1 for illustrative purposes only. The general concept can beextended in both the RF and wavelength dimensions. Each rectangle inFIG. 1 represents a “block” of service that could be provided to, forexample, a user terminal via a conventional set-top box. The servicesmay be video, audio, game, shopping or other services typically providedby entertainment service providers. Shopping services are not limited toservices for goods such as clothing and household items but may alsoinclude purchases of stocks, bonds, trusts etc. The user terminal, canthus for a modest increase in cost of components (optical filter and RFblock conversion) use his/her conventional set-top box to access anorder of magnitude more video than would otherwise be available tohim/her. The ability to provide such increases in service capacity forlow marginal cost is widely believed to be a necessary characteristicfor success in the future for telecommunication operators.

FIG. 2 a depicts the system set up for generating thewavelength-frequency plan of FIG. 1. Broadband signal source 205provides a signal to a first WDM 210. RF₁ to RF₄ to the stacks of RFcarriers modulating different wavelengths. The first WDM 210 (on theleft) separates the optical spectrum from the broadband source into fouroptical bands corresponding to the vertical columns in FIG. 1. Each ofthe RF bands is then modulated with modulator₁ to modulator₄ with acomposite signal representing the data in four independent RF blocks(shown in FIG. 2 b) corresponding to a vertical stack of RF blocks shownin FIG. 1. The combined broadcast signal at the output of the second WDM215 is distributed to the end user terminals (serviced by a remote node)through a WGR (or equivalent) in the field. It should be appreciatedthat the broadcast signal is usually optically amplified, split andconnected to multiple WGR remote nodes to achieve the maximum costsharing of the head end equipment.

FIG. 2 c depicts an alternative embodiment of the present invention. Inparticular, FIG. 2 c shows an alternative embodiment of the PONtransmitter. In this embodiment, the single broadband source 205 andfirst WDM 210 of FIG. 2 a have been replaced with N discrete broadbandsources 250. The optical bandwidth of each wavelength band is nominallyequal to or greater than the Free Spectral Range (FSR) of the WGR at theremote node. N discrete modulators (Mod 1, Mod 2, . . . , Mod N) impressRF signals onto the corresponding optical bands. The N optical bands arecombined via a WDM or passive combiner 255. The combined signal(combined N optical bands) is then propagated along a feeder fibertoward the WGR (not shown). It should be noted that the plurality of Ndiscrete broadband sources may be co-located or located at differentsites.

FIG. 3 a shows a diagram of the system concept. At central officetransmitter 305, the output of a broadband ASE source 306, for example,a gain-flattened Erbium-Doped Fiber Amplifier (EDFA) not shown, issliced into multiple optical bands whose width matches the FSR of thedistribution WGR at a Remote Node (RN) 310 (four bands are shown in theexemplary embodiment). Central office transmitter 305 is coupled to theremote node 310 in the exemplary embodiment via feeder fiber 315. Eachspectral band is modulated with multiple blocks of RF subcarriers. Inthe case of the system demonstration for the present invention, four RFblocks were derived from a commercial satellite antenna. Each RF blockof 500 MHz contained greater than 80 digital video channels multiplexedinto 16 QPSK carriers in the 950-1450 MHz band. After block-conversioninto blocks between 50-550, 550-1050, 1050-1550, and 1550-2050 MHz,these RF bands were combined to externally modulate each of the fouroptical bands. Consequently, the re-multiplexed optical signal in thefeeder fiber contained the entire service matrix shown in the inset toFIG. 3 a: each square box represents a 500 MHz block of the commercialservice.

In principle, this service suite can be delivered to a WGR at a RN whereit can be spectrally sliced and delivered to subscribers, along withswitched traffic. Here, the present invention is concerned principallywith the broadcast services. The remote node 310 is coupled to a usersite 320 in the exemplary embodiment via distribution fiber 325. Anoptical filter 330 at the user/subscriber site 320, nominally matched toone of the transmitter (optical) WDM bands, selects a “column” (stack ofRF blocks). The optical signal is detected with an optical-to-electricalconverter 335, such as an Avalanche PhotoDiode (APD) or PIN-FETreceiver, the resulting RF stack is block-converted and bandpassfiltered to send the desired RF block to the set-top box 340. Thepresent invention can be considered a new type of set-top box. Thepresent invention is identical to a conventional commercially availableset-top box, which has been modified to include a front end consistingof an optical bandpass filter and an RF block converter, as well assimple electrical circuitry to keep track of not only the conventionalchannel number but also the optical band number and RF block number. Thepresent invention effectively multiplies the number of channels that canbe accessed with the set-top.

FIG. 3 b shows an alternative embodiment of the user site 320 of thepresent invention. Specifically, a coarse WDM (CWDM) is depicted withmultiple inputs. FIG. 3 a has a small rectangular box with a singleinput interposed between the distribution fiber 325 and the small boxmarked with an “R” which represents an optical to electrical converter.FIG. 3 b, on the other hand, depicts a CWDM with a single input andmultiple outputs with one of the multiple outputs forwarded to the smallbox marked with an “R”. The CWDM of FIG. 3 b functions as ademultiplexer. The CWDM may have multiple inputs as well. The CWDM ofFIG. 3 b more clearly illustrates the function of selecting one of theoptical bands.

A problem with this scheme is that the spectral slice, the opticalportion of the column sent to the detector from its corresponding WGRoutput port, has a narrow optical width (i.e. the WGR channel width).Adding RF blocks reduces (because of clipping limitations) themodulation depth of each RF carrier, making the system susceptible tospontaneous-spontaneous beat noise N_(sp-sp). The present inventionreduces this impairment by passively splitting the broadcast signal andintroducing it to the WGR on several input ports. This multiplies theeffective optical bandwidth by the number of connected ports anddecreases the optical power only by the splitter excess loss, not itssplitting ratio. This feature, and the suppression of Mach-Zehnderinterferometric noise, accrues by virtue of routing and cyclicalproperties of the WGR. FIG. 4 shows the broadcast signal distribution toend user terminals through the WGR. The configuration associated withusing multiple input ports of the WGR is shown in FIG. 3 b. A moreconventional implementation would have the feeder fiber directlyconnected to the input port of the WGR. The present invention includesdashed and solid circles and the associated dashed and multiple solidlines, which are input to the WGR. That is, multiple input ports areused in the present invention. Heavy vertical lines represent thespectral bands defined by the system depicted in FIG. 2 a. The smallerrectangles correspond to the WDM channels defined by the WDM, which islocated near the subscriber's (user's) home terminal (serviced by aremote node). The WDM is a WGR (or equivalent) and, illustratively, theoutput fibers run to the individual subscribers' (users') terminalsserviced by a remote node. Each small rectangle is a spectral slice,located in one of the service bands, and each slice carries a replica ofthe four RF blocks that were modulated in accordance with FIG. 2 a. Itshould be realized that a coarse WDM (shown in the FIG. 3 as the firstdevice in the user's equipment) similar to those depicted in FIG. 2 a isrequired at the user end to re-segregate the multiplicity of differentservices (either broadcast or switched) into individual spectral bands.

Referring to FIG. 3 a, there is shown a central office transmitter 305,a remote node 310 and a user site 320. The signal source is shown as anASE source 306 but it may be an LED or any other equivalent signalsource. Experimental implementation of this system used passivesplitters and bulk thin-film filters to emulate the transmitter's WDMs,and used a single 2.5 Gb/s LiNbO₃ modulator to simultaneously modulatethe entire spectrum after the EDFA noise source. The noise reductionscheme was tested by connecting from one to six of the ports from a 1×8power splitter to either an 8×8 WGR with 100 GHz spacing or a 16×16 WGRwith 50 GHz spacing at the RN 310. (The ˜impairment for a single inputport with a 50 GHz WGR made the system impossible to operate.) Since thepoint of a WDM PON is the ability to upgrade to switched services, thedashed line in the RN shows how those switched services would be splitoff (selected) with a coarse WDM before being introduced to the WGR. Itis noted that this technique is similar in function to the “2 PONs in 1”approach but does not require WDMs on the output lines to re-multiplexthe broadcast and switched signals.

Satellite TV broadcast signals from a Direct Satellite Service (DSS)were applied to the system so that the RF blocks represented realisticexisting service loads. The receiver input is optically attenuated untilthe video was corrupted enough to create “blockiness” on the video or achirp on the audio channel. Thus, the receiver sensitivity for anychannel in any RE block in the service matrix could be measured. Thedelivery of “unstacked” 500 MHz blocks over several different wavelengthbands and dispersion compensation techniques for broadband signaldelivery in such systems have been demonstrated. Interest in the presentinvention is to deliver the entire service suite and to test theefficacy of the spontaneous˜spontaneous beat noise reduction technique.

FIG. 5 shows measured receiver sensitivities for each RF carrier inoptical “column 2,” a stack of RF blocks. In these experiments, six ofthe WGR input ports were fed, leaving two for switched services. Theresults are fairly uniform and decrease along the band, due partly toelectronics between the final RE mixer and the set-top box.

Whereas FIG. 5 showed the subcarriers for “column 2” only, FIG. 6 showsconsistency in going along the “rows” of blocks. That is, the groups offour symbols are fairly tight. These two figures demonstrate that withimproved RF engineering, one can expect sensitivities near −35 dBm.

Finally, FIG. 7 shows the effect of connecting multiple splitter outputsto the multiple WGR inputs. The carrier to noise ratio (CNR) for thesignals detected at the APD can easily be calculated for the presence ofthermal, shot, and sp-sp beat noise components. Considered in isolation,the CNR due to sp-sp beat noise is given by

$\begin{matrix}{{CNR}_{{sp} - {sp}} = \frac{m^{2}\kappa\; B_{o}}{{{NF}_{APD}( {1 + p} )}B_{e}}} & (1)\end{matrix}$where m is the modulation index for the QPSK subcarriers, κ is thenumber of WGR input ports connected to the splitter, B_(o) is theoptical bandwidth of a single WGR channel, NF_(APD) is the noise factordue to the APD avalanche, 0≦p≦1 is the degree of polarization (apolarizer for the modulator of the present invention was used so p=1),and B_(e) is the electrical bandwidth of a subcarrier (30 MHz).Decreasing m (increased number of RF carriers) and B_(o) (spectralslicing) ultimately make CNR_(sp-sp) approach the minimum system CNR. Atthis point, equation (1) shows that increasing optical power does nothelp, but increasing effective optical bandwidth does. After a baselinesensitivity test (with WGR removed to obtain low sp-sp noise) thepenalties associated with connecting multiple ports were measured andcalculated. Expressed as an equivalent optical bandwidth, the theory isa universal curve. As can be seen, there is excellent agreement betweenthe theory and experiment for both WGRs (1-6 ports for the 100 GHz WGR,and 2-6 ports for the 50 GHz WGR).

The broadcast delivery of 1280 digital video channels on a WDM PON hasbeen described and demonstrated. While any services may be deliveredbesides video and only one-way services have been described herein. Thepresent invention may be used to deliver two-way or interactive servicesas well. Four wavelength-division multiplexed ASE bands, each modulatedwith 64 QPSK subcarriers, were spectrally sliced at the RN's WGR. Whilethese numbers of carriers were demonstrated, a lesser or greater numbermay be considered without departing from the spirit of the presentinvention. When only one input port of a 50-GHz WGR was connected, therelatively low optical modulation depth per QPSK subcarrier (required toavoid nonlinear distortion) in conjunction with spontaneous-spontaneousbeat noise (caused by the decrease in optical bandwidth due to thefiltering associated with the WGR) resulted in an infinite power penaltyat the receiver due to spontaneous-spontaneous beat noise. This effectwas suppressed with a novel remote node architecture, which increasedthe effective optical bandwidth.

It should be clear from the foregoing that the objectives of theinvention have been met. While particular embodiments of the presentinvention have been described and illustrated, it should be noted thatthe invention is not limited thereto since modifications may be made bypersons skilled in the art. The present application contemplates any andall modifications within the spirit and scope of the underlyinginvention disclosed and claimed herein.

1. A system for delivering a plurality of video blocks to a userterminal by a remote node, comprising: a plurality of broadband signalsources for providing a plurality of broadband signals, wherein eachbroadband signal is comprised of a plurality of optical bands; aplurality of modulators, wherein each of the plurality of modulatorsimpress one of the optical bands with a composite signal representingdata in a plurality of independent RF blocks to form a plurality ofmodulated signals; a device for combining the plurality of modulatedsignals to form a combined broadcast signal; a feeder fiber coupled tothe remote node for delivering the combined broadcast signal to theremote node; and a distribution fiber for distributing the combinedbroadcast signal to the user terminal.
 2. The system according to claim1, wherein said optical bands nominally match a Free Spectral Range(FSR) of a Waveguide Grating Router (WGR) at said remote node.
 3. Thesystem according to claim 2, wherein an optical fiber is used to selecta stack of RF blocks.